Abdominal aortic aneurysm (AAA) is a progressive enlargement of the abdominal aorta with a prevalence of ~9% in the male population of age 65 years and above. The disease accounts for over 15,000 deaths each year in the US alone. Moreover, the only treatment option for AAA is surgical intervention, which comes with an associated financial burden. With 25,000 surgical repairs occurring in the US annually, the total expenditure runs into billions of dollars thus impacting economic burden of the society. Only with an increased understanding of the mechanism(s) underlying the development of AAA will novel non- surgical therapies be devised to reduce AAA progression. While multiple factors are involved in disease pathogenesis, the critical stimulus is the differentiation and infiltration of nave macrophages (Mf). A typical response to aortic injury elicits differentiation of a classic M form of macrophages from Mf, which produce high levels of pro-inflammatory cytokines and chemokines to phagocytize apoptotic cells and damage- associated debris. However, their uncontrolled production damages the healthy tissue leading to aortic dilation and death of vascular cells. In contrast, Mfs have also the potential to be programmed to an alternative differentiation termed M2-like macrophages. These M2 macrophages have the ability to inhibit the inflammatory response and promote tissue repair. Although the roles of pro-inflammatory M1 macrophages have been well characterized in various vascular diseases including AAA, little is known about the critical factors that initiate the differentiation of Mf into M2-phenotyp and their potential roles in AAA. Our lab has recently identified Notch1 as one such which factor promotes M2-differentiation of Mf. The long term goal of our research is to determine the roles for M2-differentiated macrophages in vascular diseases. The objective of this proposal is to determine if Notch1 deficiency in macrophages prevents AAA progression by increasing M2 differentiation of Mf. Based upon our preliminary studies and published literature, we hypothesize that deficiency of Notch1 will increase differentiation of Mf into M2-phenotype by TGF-2 dependent mechanisms. We plan to test our hypotheses with the following Specific Aims: i) Determine if myeloid-specific inducible- Notch1 haploinsufficiency can prevent AAA progression; ii) Determine if M2-differentiated macrophages protect against AAA development in mice and iii) Determine if TGF-2 is a critical requirement for Notch1-mediated M2 differentiation of Mf. These Specific Aims are designed to provide a comprehensive assessment of the regulatory role of Notch1 signaling on M2 differentiation of Mf in the setting of AAA. Our approach is innovative as we are proposing a series of experiments using double mutant mice generated in the lab, cell culture and human aortae obtained from AAA patients to achieve our goal. The proposed research is highly significant because it may provide the necessary steps for the therapeutic application targeting Notch1 to increase M2 population of macrophages for the prevention of AAA growth.